Process for producing a cast metal strip, and two-roll casting device used for this process

ABSTRACT

The invention relates to a method for producing a cast metal strip using a twin roll casting installation comprising two casting rolls and two lateral plates that together define a molten metal chamber and a casting gap. Molten metal is introduced in to the molten metal chamber and forms the molten metal chamber a molten metal bath with a bath surface that is open to the top. A cast metal strip is conveyed from the molten metal chamber and through the casting gap. A delimiting surface area for collecting particles that are foreign to the molten metal is produced on the surface of the bath under the effect of at least one gas jet. The aim of the invention is to substantially avoid the introduction of particles that are foreign to the molten metal into the surface or into the near-surface zone of the cast strip. For this purpose, the at least one gas jet is directed together with the casting roll onto the bath surface at a distance of the gas jet axis to the contact line of the bath surface.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a 35 U.S.C. §§ 371 national phase conversionof PCT/EP2004/004947, filed 10 May 2004, which claims priority of GermanApplication No. A 772/2003, filed 19 May 2003. The PCT InternationalApplication was published in the German language.

BACKGROUND OF THE INVENTION

The invention relates to a process for producing a cast metal stripusing two casting rolls and two side plates, which together form a meltspace and a casting gap, metal melt being fed into the melt space and inthe melt space forming a melt bath with a bath surface which is open atthe top, and a cast metal strip being delivered out of the melt spacethrough the casting gap, and a delimited surface region for thecollection of particles which are foreign to the melt being formed onthe bath surface under the action of at least one gas jet, and to atwo-roll casting device used for this process.

The invention preferably relates to a casting process for producing acontinuously cast steel strip with a strip thickness of between 0.5 mmand 10 mm using a two-roll casting installation, with the cast steelstrip being removed substantially vertically downward.

A two-roll casting device with a vertically delivered metal strip isgenerally known and comprises, as is diagrammatically illustrated inFIGS. 1 and 2, two driven, oppositely rotating casting rolls 1, 2 andtwo sides plates 3, 4, which are preferably placed against the end sidesof the casting rolls and thereby form a melt space 5 for receiving metalmelt introduced through a submerged casting nozzle 6. The two axes ofrotation of the casting rolls lie in a horizontal plane and are arrangedparallel to and at a distance from one another, so that a casting gap 7is formed between the casting rolls; the longitudinal extent of thiscasting gap 7 is delimited by the side plates, and therefore the castinggap 7 has a cross section which corresponds to the cross section of thedesired cast strip. With continuous supply of metal melt into the meltspace, a melt bath with a bath surface 8 that is open at the top isformed therein. Above the bath surface, the melt space is delimited by acovering hood 9, which bears, either so as to form a seal or leavingclear a gap, against the casting rolls and side plates, in order tosubstantially prevent the access of external air. At the bottom, themelt space opens out into the casting gap, from which the metal stripemerges. When the casting rolls are rotating, starting from the contactlines 10, 11 between the bath surface and the cooled casting rolls, twostrand shells 12 are formed on the lateral surfaces of the casting rollswhere they enter the melt bath, the strand shells becoming continuouslythicker and ultimately being combined in the casting gap to form themetal strip 13.

With a continuous supply of metal melt into the melt bath through thesubmerged casting nozzle, which causes movement in the melt bath,nonmetallic particles which are foreign to the melt are entrained. Theseparticles float to the surface of the bath, where they agglomerate,together with particles which are foreign to the melt and were generatedin the mold melt bath by chemical reaction with refractory material orby reoxidation, and are incorporated in the strand shells predominantlyat the contact line with the casting rolls directly at the lateralsurface of the casting rolls, forming inclusions and seeds formacrocracks and microcracks at the surface and in the region close tothe surface of the cast metal strip.

A two-roll casting installation and a casting process for casting ametal melt in accordance with the prior art described is known, forexample, from JP-A 2001-314946, WO 02/083343 and JP-A 2-207946.

To keep particles which are foreign to the melt away from the contactline between the casting-roll surface and the bath level surface, it isproposed in JP-A 2001-314946 that gas jets be applied in the region ofthis contact line, causing the particles which are foreign to the meltto drift away toward the center of the melt pool. The gas jets coverpart of the casting roll surface and an edge region of the bath levelsurface, but bath fluctuations and temperature fluctuations whichinfluence the strand shell growth occur at the casting roll surface in asensitive area depending on the intensity and temperature of the gasjets. Unfortunately, substantially uniform starting conditions for theformation of the strand shells in this region are particularly importantfor the end product.

According to WO 02/083343, drifting of particles which are foreign tothe melt and have been entrained into the melt bath toward the contactline between the metal bath and the lateral surfaces of the castingrolls is avoided during casting operation by means of shields which areobliquely immersed in the metal bath and the lower edges of which arepositioned below the level of the outlet openings of the submergedcasting nozzle. The intention of this is to additionally create a meltpool in the melt space, in which the nonmetallic particles can beseparated off. The metal strip which is produced continuously using thetwo-roll casting device is wound into coils, and at the end of thewinding operation of each individual coil, the shields are removed fromthe metal bath and the particles which have been separated out at thesurface of the bath are blown toward at least one of the casting rollsurfaces using gas nozzles and in this way discharged together with ashort piece of the metal strip. The main drawback of this process isthat each cast coil produces a piece of scrap, which interrupts thecontinuous production process and increases the scrap rate ofproduction. Furthermore, metal melt accumulates on the shields andsolidifies each time the shield is raised. If the shield consists ofrefractory material, eroded particles of the refractory material areadditionally introduced into the melt, or chemical reactions occurbetween the liquid steel and the refractory material, which produceadditional impurities.

JP-A 2-207946 has disclosed a two-roll casting device in which theforeign particles floating on the bath surface are removed by beingcontinuously scooped out using rotating cup mechanisms. Since thesedevices at the bath surface have to work at the melting point of themetal, there is likely to be a high number of operating faults in thesemechanical devices. In addition, in the case of a steel bath, the bathsurface has to be protected from contact with atmospheric oxygen, andconsequently it is not feasible to use scoop devices of this type underthese conditions.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to avoid thedrawbacks of the prior art described and to propose a process forproducing a cast metal strip and a two-roll casting device, in which theintroduction of particles which are foreign to the melt at or into thesurface or into the region close to the surface of the cast strip issubstantially avoided, a contact line between the bath surface and thecasting roll lateral surface which is substantially free of disruptionand is delimited from the formation of any waves at the bath surface isachieved and at the same time contact of oxygen with the bath surface isas far as possible avoided.

Working on the basis of a process of the type described in theintroduction, this object is achieved by virtue of the fact that the atleast one gas jet is directed on to the bath surface with the gas jetaxis at a distance from the contact line between the bath surface andthe casting roll.

In this case, the at least one gas jet is shaped in such a way that nogaps through which particles which are foreign to the melt can escaperemain along the delimited surface region. In general, the delimitedsurface region may be formed by a gas jet which forms a closed ring withany desired outer contour or by a plurality of successive gas jets. Atthe same time, in particular in the case of metal melts which have ahigh tendency to oxidation, such as steel, an inert or reducingshielding gas atmosphere is produced and maintained above the metal bathand within a melt space which is optimally closed off with respect tothe ingress of external air, which virtually rules out reoxidation ofthe metal melt.

The at least one gas jet is directed directly on to the bath surface.This produces a calm edge strip, which remains substantially unaffectedby the formation of waves at the bath surface, between the region ofcontact between the gas jet and the bath surface and the casting rollsand/or side plates which delimit the melt space. This measure greatlyassists with a constant, uniform and undisturbed formation of strandshells at the lateral surfaces of the casting rolls which rotate inaccordance with the casting speed, if the casting roll surfaces also runand function in an optimally stable and homogenously uniform way.

In this context, it is particularly expedient if the at least one gasjet is directed on to the bath surface at an angle from 25° to 145°,preferably at an angle of from 35° to 90°, based on a horizontal plane.In this case, the bath surface substantially corresponds to thishorizontal plane.

Each gas jet is assigned a gas jet axis. Preferably, the at least onegas jet is directed on to the bath surface with the gas jet axis at adistance from the contact line between the bath surface and the castingroll and/or from the contact line between the bath surface and the sideplate. This distance is preferably constant and in a range between 10 mmand 50 mm, measured on the bath surface.

Since the side plates, unlike the rotating casting rolls, aresubstantially stationary, the at least one gas jet can be directed on tothe side plate surface at a distance from the contact line between thebath surface and the side plate, and at least a part-stream of the gasjet is effectively diverted on to the bath surface.

The gas jet or gas jets are preferably in the form of fan jets andemerge from a correspondingly shaped nozzle. It is expedient for amultiplicity of nozzles to be arranged in succession, so as to produce acontinuous narrow gas jet, similar to that used in a gas meter.

To form a delimited surface region of any desired shape on the bathsurface, the at least one gas jet is in the form of a partially curvedfan jet.

Once it emerges from the gas jet nozzle, the gas jet diverges with anopening angle of between 10° and 35° in the direction of flow. For theuniform and stable formation of a strand shell, it is necessary for allof the diverging gas jet to strike the bath surface, rather than beingpartially directed on to the lateral surface of the casting roll. At theside plates, which may execute an oscillating movement, direct contactbetween the gas jet and the side plate is perfectly permissible, sincethe disadvantageous effects encountered at the lateral surfaces of thecasting rolls do not occur here.

According to a preferred embodiment, between the two side plates, ifappropriate leaving clear a distance with respect to the side plates,the at least one gas jet acts on the bath surface parallel or obliquely,without interruption, to the contact line between the bath surface andthe casting roll. This ensures that the casting roll surface iscontinuously shielded from contact with particles which are foreign tothe melt. Continuous discharge of the particles toward the side platesand therefore into the edge zone of the cast metal strip is possible andalso desirable, since the cast metal strip, at least before it is woundin a downstream coiler, passes through a trimming station, which is notnecessarily arranged within the actual two-roll casting installation,and therefore a controlled increase in the level of nonmetallicinclusions in this region does not cause any additional scrap material.Arranging the gas jet so as to run obliquely with respect to the contactline between the bath surface and the casting roll additionally promotescontinuous discharge of particles which are foreign to the melt towardthe side plates. Furthermore, leaving clear a distance with respect tothe side plates avoids local cooling of a spatially restricted zone atthe side plates by the gas jets.

Equally, between the two casting rolls, if appropriate leaving clear adistance with respect to the casting rolls, the at least one gas jetacts on the bath surface parallel, without interruption, to the contactline between the bath surface and the side plate. As a result, if noincrease in particles foreign to the melt is desired even at the edgesof the metal strip while casting operation is ongoing, suitableshielding is achieved. Leaving clear a distance with respect to thecasting rolls avoids local cooling on the casting roll lateral surfacealong a circumferential strip and therefore different levels of strandshell growth along the contact line between the casting roll lateralsurface and the bath surface.

A further improvement to the restricting of the particles foreign to themelt is achieved if at least in sections at least two gas jets act onthe bath surface at a distance from one another. This measure improvesthe surface quality of the strip in particular along the contact linebetween the casting roll lateral surface and the bath surface. It ispreferable for the two gas jets to be arranged equidistantly withrespect to one another.

Components of the two-roll casting device which form the melt space orare arranged directly within it can be included when forming thedelimited surface region with gas jets. In this case, the delimitedsurface region is formed in sections by at least one gas jet and insections by sections of the side plates or the casting rolls or asubmerged casting nozzle or other internal fittings.

It is preferable for the at least one gas jet which strikes the metalbath at an angle to form a gap-free bow wave, i.e. a swell at the bathsurface which extends parallel to the direction of extent of a fan jetand encloses the delimited surface region at least in sections. The bowwave may be continuous and in this way form this delimited surfaceregion, or may form a delimited surface region in combination withcomponents of the two-roll casting device, such as sections of the sideplates or of the casting rolls or of a submerged casting nozzle or ofother internal fittings.

The bow wave formed by the gas jets is held substantially constant at aheight of from 0.05 mm to 10 mm, preferably from 0.1 mm to 3 mm, abovethe normal level of the bath surface. This creates a collection tank forthe particles which are foreign to the melt, and the particles are heldthere until they are discharged in a controlled way or until castingends automatically.

An inert or reducing gas is used to form the gas jet, to ensure thatthere is no reoxidation of the metal melt at the bath surface in thisregion. Preferred gases which can be used include argon, nitrogen, N+H₂or mixtures of at least two of these gases.

In the starting phase of a casting process, the process according to theinvention should only be deployed when an operating bath level has beenreached and therefore the metal melt has been substantially stabilizedand calmed in the melt space and in particular at the bath surface.Therefore, during the starting phase of the casting process, the actionof at least one gas jet on the bath surface is expediently only switchedon 10 sec to 2 min after the introduction of melt into the melt spacehas commenced (start of casting).

Over a prolonged casting period, particles which are foreign to the meltaccumulate within the delimited surface region and have to be removed atleast at periodic intervals. This is preferably done duringinterruptions to production for operation reasons, during which the meltspace itself is completely emptied and then the installation isrestarted and casting recommenced. If these time intervals are too long,the action of at least one gas jet on the bath surface is interrupted insections in a time interval in order for accumulated particles which areforeign to the melt to be discharged from a delimited surface region.This is achieved by the action of at least one gas jet on the bathsurface being interrupted either along the contact line between the bathsurface and at least one of the two casting rolls or along the contactline between the bath surface and at least one of the two side plates,and preferably along the contact line between the bath surface and bothside plates. The discharge of particles which are foreign to the melttoward the side walls and therefore into the edge region of the castmetal strip avoids the formation of inclusions close to the surface atthe wide sides of the metal strip, and this edge strip with increasedlevels of inclusions is removed during the trimming of the strip, whichtakes place within a subsequent process step. The discharging ofparticles which are foreign to the melt via the contact surface betweenthe casting rolls and the metal melt in the melt space expediently takesplace in a time interval immediately after the coil weight of the castmetal strip has been reached.

The invention also proposes a two-roll casting device for producing acast metal strip of the generic type described in the introduction,having two casting rolls driven in rotation and side plates, which bearagainst the end sides of the casting rolls, these casting rolls and sideplates together forming a melt space for receiving a melt bath with abath surface, and a casting gap. At least one gas jet nozzle with anoutlet opening for a directed gas jet is arranged in the melt space ordirected or projecting into the melt space, in such a way that adelimited surface region for collection of particles which are foreignto the melt is formed on the bath surface. A two-roll casting deviceformed in this way is characterized in that the outlet opening of thegas jet nozzle is directed directly on to the bath surface at a distancefrom the contact line between the bath surface and the casting roll.

At a distance above the bath surface, the melt space is protected fromthe ingress of air by a covering hood. The covering hood bears againstthe side plates and the casting rolls with a contact surface or a seal,or in particular is set at a narrow gap from the casting rolls, in whichcase shielding gas which is introduced into the melt space escapesthrough these gaps and in this way prevents external air from enteringthis melt space. At least the outlet openings of the gas jet nozzlesproject through the covering hood into the melt space and are preferablysecured to the covering hood and oriented.

In general, the orientation of the outlet opening of the gas jet nozzlesdetermines the direction of the emerging gas jet. To this extent, theorientation of the nozzle axis in the outlet cross section of the gasjet nozzle corresponds to the orientation of the gas jet axis of the gasjet in the cross section of the outlet opening. Since the outletopenings of the gas jet nozzle and therefore the defined nozzle axis inthe outlet opening of the gas jet nozzle are directed directly on to thebath surface, the drifting of particles which are foreign to the meltinto undesirable zones of the bath surface is avoided. Favorableconditions for this are achieved if the distance between the gas jetaxis directed on to the bath surface and the contact line between thebath surface and the casting roll is in a range from 10 mm to 50 mm,measured on the bath surface. Favorable conditions likewise result ifthe outlet opening of the gas jet nozzle or the nozzle axis, in theoutlet cross section of the outlet opening, is directed toward the bathsurface at an angle of from 25° to 145°, preferably at an angle of from35° to 90°, based on a horizontal plane. The bath surface in this caseforms the horizontal plane.

To produce a very narrow but elongate gas jet, the gas jet nozzle isconfigured as a fan jet nozzle or slot nozzle with a slot-shaped outletopening. Arranging a plurality of gas jet nozzles of this type insuccession allows a delimited region of any desired shape to be enclosedon the bath surface using gas jets.

It is expedient for the outlet opening of the gas jet nozzle to bedirected directly on to the bath surface at a distance from the contactline between the bath surface and the side plate.

A beneficial effect is produced if, between the two side plates, ifappropriate leaving clear a distance with respect to the side plates,the outlet opening of the gas jet nozzle is directed on to the bathsurface parallel to the contact line between the bath surface and thecasting roll.

Excessive local cooling at the side plates under the action of acontinuous gas jet is avoided if, between the two casting rolls, ifappropriate leaving clear a distance with respect to the casting rolls,the outlet opening of the gas jet nozzle is directed on to the bathsurface parallel to the contact line between the bath surface and theside plate. Excessive local cooling at the casting roll surface isavoided if, between the two casting rolls, if appropriate leaving cleara distance with respect to the casting rolls, the outlet opening of thegas jet nozzle is directed on to the bath surface parallel to thecontact line between the bath surface and the side plate.

Improved shielding with respect to the particles which are foreign tothe melt is achieved if a gas jet nozzle is equipped with two,substantially equidistant, outlet openings for targeted gas jets, or twogas jet nozzles each having one outlet opening are provided, in whichcase the outlet openings are arranged in such a way that adouble-delimited surface region for the collection of particles whichare foreign to the melt is formed on the bath surface.

A continuous, delimited region for the collection of particles which areforeign to the melt is achieved if the outlet openings of at least onegas jet nozzle are directed on to the bath surface in such a way that,under the action of gas jets, they form a delimited surface region onthe bath surface. However, this is also possible if the outlet openingsof at least one gas jet nozzle are directed on to the bath surface insuch a way that, together with sections of the casting rolls or of theside plates or of other internals in the melt bath, and under the actionof gas jets in sequence, they form a delimited surface region on thebath surface.

Further advantages and features of the present invention will emergefrom the following description of non-restricting exemplary embodiments,in which reference is made to the appended figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a two-roll casting device according to the prior art incross section through the casting rolls,

FIG. 2 shows a two-roll casting device according to the prior art inplan view,

FIG. 3 shows a two-roll casting device having the casting nozzlesaccording to the invention or gas jets directed in accordance with theinvention,

FIG. 4 shows the gas jet nozzle orientation and gas jet orientation onto bath surface according to one embodiment of the invention,

FIG. 5 shows the formation of a delimited surface region on the bathsurface according to one embodiment of the invention,

FIG. 6 shows the formation of a delimited surface region on the bathsurface according to a further embodiment,

FIG. 6A shows an embodiment in which the delimited surface region isformed and in which gas jets strike surfaces of the side plates.

FIG. 7 shows the incorporation of the gas jet nozzles in the coveringhood,

FIG. 8 shows the arrangement of a delimited surface region on the bathsurface with double gas jets,

FIG. 9 shows a gas jet nozzle with two outlet openings.

FIG. 10 illustrates an embodiment of the invention in which the gas jetsstrike the surface of the bath.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The basic structure of a two-roll casting device has already beendescribed in the summary of the prior art with reference to FIGS. 1 and2. The reference numerals which have already been introduced to certaincomponents in those figures are also applied accordingly for the samecomponents in the text which follows. Two-roll casting devices are usedfor the continuous production of continuous-cast steel strips.

In particular for stainless steel grades, particularly high demands areimposed on the surface quality of the strips produced, since even minorinclusions of foreign substances, such as slags, metal oxides and thelike, at the surface or in the region close to the surface form seedcells for microcracks and macrocracks, with noticeable adverseconsequences for the surface condition.

The principle on which the process according to the invention is basedis illustrated in FIG. 3. A melt space 5, in which there is steel meltwhich is supplied continuously via a submerged casting nozzle 6, isformed between two casting rolls 1, 2, which rotate in the directionindicated by the arrows, and side plates 3, which bear against the endsides of the casting rolls and only one of which is illustrated in thissectional illustration. The melt bath forms a bath surface 8 whichextends between the two casting rolls 1, 2. Starting from the contactlines 10, 11 between the bath surface 8 and the casting roll surfaces14, 15 of the internally cooled casting rolls 1, 2, strand shells 12 areformed and are fused together in the casting gap 7 to form the metalstrip 13.

Gas jet nozzles 16 are arranged at a distance above the bath surface 8,with their outlet openings 17 or their nozzle axes 18 in the outletcross section of the outlet opening 17 directed obliquely toward thebath surface 8. The gas jets 20 which emerge with the gas jet axes 21produce a bow wave 24 of a certain height on the bath surface 8. Thisheight also is determined to a significant extent by the flow velocityof the gas jets and the pressure with which they strike the bathsurface. Particles which are foreign to the melt and float on the meltbath accumulate between opposite bow waves 24 or within the surfaceregion 30 which is delimited by a bow wave. The gas jet nozzles 16 areconnected to supply lines 26, through which they are supplied with aninert or reducing gas. A multiplicity of gas jet nozzles are connectedto the supply lines, which preferably form a circular pipeline.

In FIG. 4, the outlet opening 17 or the nozzle axis 18 of the gas jetnozzle 16 is directed on to the bath surface 8, so that the gas jets 20strike the bath surface directly and produce a bow wave 24. In thiscase, the outlet opening 17 or the gas jets 20 or the gas jet axes 21is/are directed toward the bath surface 8, which defines a horizontalplane E at an angle α which may be between 25° and 145°. The angle α isin this case determined from the casting roll side, as illustrated inFIG. 4.

A multiplicity of gas jets which are generated by gas jet nozzlesarranged in a row produce a delimited surface region on the bathsurface, within which surface region the particles which are foreign tothe melt are accumulated. FIG. 5 shows the bath surface 8 between twocasting rolls 1, 2 and two side plates 3, 4. Above the bath surface 8,gas jet nozzles 16 are positioned parallel to the casting rolls andparallel to the side plates, generating targeted gas jets 20 directedtoward the bath surface 8. They enclose a substantially rectangulardelimited surface region 30 on the bath surface 8, in which theparticles which are foreign to the melt accumulate.

FIG. 6 illustrates a further advantageous embodiment for forming twodelimited surface regions 30. In this case, gas jet nozzles 16 areoriented in an angular position with respect to the casting rolls 1, 2and accordingly form a bow wave which is oriented obliquely with respectto the casting rolls. The submerged casting nozzle 6, which is centrallysubmerged in the melt bath, is included in the formation of thedelimited surface region 30 and delimits this surface region in asubsection. In a further subsection, the two surface regions 30 arerespectively delimited by the side plates 3, 4. The approximatelyV-shaped formation of the two delimited surface regions 30 allows theparticular advantage of continuous discharge of particles which areforeign to the melt toward the side plates 3, 4 and therefore into theoutermost edge regions of the cast steel strip.

As illustrated in FIG. 6A, the gas jet nozzles 16 may be oriented suchthat the gas jets strike the surfaces of side plates 3, 4.

One possible embodiment for the incorporation of gas jet nozzles intothe covering hood 9 which shields the melt bath from the ingress ofexternal air is illustrated in FIG. 7. Between the casting rolls 1, 2the covering hood 9 is positioned between the casting roll surfaces 14,15, at a short distance therefrom, with supports (not illustrated inmore detail) above the bath surface 8. The covering hood 9 is equippedwith apertures or edge-side recesses, of which only one such passage 31,into which a gas jet nozzle 16 is fitted and screwed to a bracket 32 onthe covering hood 9, is illustrated here. The gas jet nozzle 16 isdesigned as a slot nozzle or fan jet nozzle with a slot-shaped outletopening 17 and has an outlet passage 19 which is straight at least inthe end region. This produces a very narrow, focused gas jet 20 which isdirected on to the bath surface 8 and forms the bow wave 24.

A further advantageous embodiment for forming a delimited surface region25 is illustrated in FIG. 8. Gas jet nozzles 16 are arranged at adistance from the bath surface 8 and its edges toward the casting rolls1, 2 and the side plates 3, 4 on all sides, with their outlet openingsdirected on to the bath surface. Two rows of gas jet nozzles 16 a, 16 b,. . . , which form gas jets 20 a, 20 b, . . . running parallel to oneanother and illustrated in FIG. 9, are oriented parallel to one anotherin a subsection along the delimited surface region along thelongitudinal extent of the casting rolls. Gas jet nozzles with twooutlet openings can also be used to the same effect. In both cases, adouble bow wave is produced. FIG. 9 shows a gas jet nozzle 16 with twooutlet openings 17 a, 17 b and with outlet passages 19 a, 19 b whichdiverge in the gas direction of flow. However, the outlet passages mayalso run parallel to one another. Two bow waves 24 a, 24 b are producedon the bath surface 8 at a distance from one another, thereby producinga double barrier to the particles which are foreign to the melt.

FIG. 10 illustrates that the gas jet nozzles 16 are arranged so that thegas jets strike the bath surface 8 but avoid directly striking thecasting rolls 1, 2.

However, the invention is not restricted to the embodiments illustratedand described, but rather can be modified in numerous ways. It is alsopossible for gas jets which follow one another and form a delimitedsurface region, as well as the associated gas jet nozzles, to bearranged in such a way that the gas jets are directed directly towardthe bath surface in one peripheral section of the delimited surfaceregion and are directed on to the casting roll surface or the sideplates in a further section.

1. A method for producing a cast metal strip from a melt space fed by ametal melt, two opposing casting rolls and two side plates at oppositeends of the two opposing casting rolls together defining and enclosingthe melt space and further defining a casting gap leading out of themelt space, the method comprising: feeding the metal melt into the meltspace for forming in the melt space a melt bath with a bath surface openon top, and delivering the cast metal strip out of the melt spacethrough the casting gap; forming a delimited surface region on the bathsurface for collection of particles foreign to the metal melt beingformed, the forming of the delimited surface region performed under anaction of at least one gas jet directed onto the bath surface, the atleast one gas jet having a jet axis that intersects the bath surface ata distance from a first contact line between the bath surface and one ofthe casting rolls, wherein an entirety of the at least one gas jetavoids directly striking the casting rolls and the at least one gas jetstrikes the bath surface with the jet axis at the distance of from 10 mmto 50 mm, measured on the bath surface, from the first contact line. 2.The method as claimed in claim 1, further comprising directing the atleast one gas jet toward the bath surface at an angle (α) of from 25° to145°.
 3. The method as claimed in claim 1, further comprising directingthe at least one gas jet onto the bath surface with the gas jet axisintersecting the bath surface at a distance from a second contact linebetween the bath surface and one of the two side plates.
 4. The methodas claimed in claim 3, wherein the distance from the second contact lineis between 10 mm to 50 mm, measured on the bath surface.
 5. The methodas claimed in claim 3, further comprising directing the at least one gasjet onto a surface of one of the two side plates at a distance from thesecond contact line, and effectively diverting at least a part-stream ofthe gas jet onto the bath surface.
 6. The method as claimed in claim 1,wherein the at least one gas jet comprises a fan jet.
 7. The method asclaimed in claim 6, wherein the at least one gas jet comprises apartially curved fan jet.
 8. The method as claimed in claim 1, whereinthe at least one gas jet diverges with an opening angle (γ) of between10° and 35° in the direction of flow.
 9. The method as claimed in claim1, wherein between the two side plates, the at least one gas jet isdirected to the bath surface parallel to or obliquely to the firstcontact line without interruption.
 10. The method as claimed in claim 3,wherein between the two casting rolls, the at least one gas jet acts onthe bath surface parallel to, without interruption, the second contactline between the bath surface and one of the two side plates.
 11. Themethod as claimed in claim 1, wherein at least in sections, the at leastone gas jet includes a first gas jet and a second gas jet, and the firstgas jet acts on the bath surface at a distance from the second gas jeton the bath surface.
 12. The method as claimed in claim 1, wherein theat least one gas jet is directed so as to form a bow wave at the bathsurface, the bow wave being formed to enclose the delimited surfaceregion at least in sections and the bow wave being kept constant at aheight above the normal level of the bath surface.
 13. The method asclaimed in claim 1, wherein the at least one gas jet comprises an inertgas or a reducing gas, or a mixture comprising the inert gas and thereducing gas.
 14. The method as claimed in claim 1, further comprisingduring a starting phase of the method, switching on the action of the atleast one gas jet on the bath surface for 10 sec. to 2 min. afterintroduction of the metal melt into the melt space.
 15. The method asclaimed in claim 1, further comprising interrupting the action of the atleast one gas jet on the bath surface in sections in a time intervalduring which particles foreign to the metal melt are discharged from adelimited surface region of the bath surface.
 16. The method as claimedin claim 15, wherein the action of the at least one gas jet on the bathsurface is interrupted along the first contact line.
 17. The method asclaimed in claim 15, further comprising directing the at least one gasjet onto the bath surface with the gas jet axis intersecting the bathsurface at a distance from a second contact line between the bathsurface and one of the two side plates, wherein the action of the atleast one gas jet on the bath surface is interrupted along the secondcontact line.
 18. The method as claimed in claim 15, further comprisingremoving particles foreign to the metal melt from the metal strip bytrimming the edges of the cast metal strip after casting thereof. 19.The method as claimed in claim 15, further comprising removing particlesforeign to the metal melt during a time interval immediately after aselected coil weight of the cast metal strip has been reached, and whilethis metal strip section which is enriched with particles foreign to themetal melt is being removed.
 20. The method as claimed in claim 2,wherein the at least one gas jet is directed toward the bath surface atan angle (α) of from 35° to 90°.
 21. The method as claimed in claim 12,wherein the bow wave is at a height of from 0.05 mm to 10 mm.
 22. Themethod as claimed in claim 12, wherein the bow wave is at a height offrom 0.1 mm to 3 mm.
 23. The method as claimed in claim 13, wherein theat least one gas jet comprises argon or nitrogen or N+H₂ or mixtures ofat least two of the foregoing.
 24. The method as claimed in claim 17,wherein the interruption is along contact lines between the bath surfaceand both of the two side plates.
 25. A two-roll casting device forproducing from a melt bath fed by a metal melt a cast metal stripcomprising: two opposing casting rolls driven in rotation, the twoopposing casting rolls having opposite end sides; side plates bearingagainst the end sides of the casting rolls; the casting rolls and theside plates positioned and configured to define together and to enclosea melt space for holding therein the melt bath with a bath surface andalso to define a casting gap; at least one gas jet nozzle having anoutlet opening and operable to provide a targeted gas jet, the nozzlebeing arranged in the melt space or being directed into the melt spacesuch that a delimited surface region for collection of particles foreignto the metal melt is formed on the bath surface, the outlet opening ofthe at least one gas jet nozzle being directed onto the bath surface ata distance from a first contact line between the bath surface and one ofthe casting rolls, such that the gas jet strikes the bath surface, thegas jet having an axis and the gas jet axis being directed to provide adistance between the gas jet axis at the bath surface and the firstcontact line, wherein an entirety of the gas jet avoids directlystriking the casting rolls and the at least one gas jet strikes the bathsurface with the jet axis at the distance of from 10 mm to 50 mm,measured on the bath surface, from the first contact line.
 26. Thetwo-roll casting device as claimed in claim 25, wherein the outletopening of the at least one gas jet nozzle is directed toward the bathsurface at an inclined angle (α).
 27. The two-roll casting device asclaimed in claim 25, wherein the outlet opening of the at least one gasjet nozzle is directed onto the bath surface at a distance from a secondcontact line between the bath surface and a side plate of the sideplates.
 28. The two-roll casting device as claimed in claim 27, whereinthe distance between the gas jet axis directed onto the bath surface andthe second contact line is in a range from 10 mm to 50 mm, measured onthe bath surface.
 29. The two-roll casting device as claimed in claim25, wherein the outlet opening of the at least one gas jet nozzle isdirected onto a side plate of the side plates at a distance from asecond contact line between the bath surface and the side plate.
 30. Thetwo-roll casting device as claimed in claim 25, wherein between the sideplates, the outlet opening of the at least one gas jet nozzle isdirected onto the bath surface parallel to the first contact line. 31.The two-roll casting device as claimed in claim 25, wherein between thetwo opposing casting rolls, the outlet opening of the at least one gasjet nozzle is directed onto the bath surface parallel to a secondcontact line between the bath surface and a side plate of the sideplates.
 32. The two-roll casting device as claimed in claim 25, whereinthe at least one gas jet nozzle comprises a fan jet nozzle with aslot-shaped outlet opening.
 33. The two-roll casting device as claimedin claim 25, wherein the at least one gas jet nozzle includes two outletopenings for providing targeted gas jets, or two gas jet nozzles eachhaving one outlet opening, such that the outlet openings are positionedand configured to form a double-delimited surface region for thecollection of particles foreign to the metal melt on the bath surface.34. The two-roll casting device as claimed in claim 25, wherein theoutlet opening of the at least one gas jet nozzle is directed onto thebath surface such that it cooperates together with sections of the twoopposing casting rolls or of the side plates within the melt space toform the delimited surface region on the bath surface under an action ofthe targeted gas jet.
 35. The two-roll casting device as claimed inclaim 25, further comprising a covering hood shaped and positioned suchthat the melt space formed by the casting rolls and the side plates isclosed off with respect to ingress of air by the covering hood; and theoutlet opening of the at least one gas jet nozzle opens out into themelt space.
 36. The two-roll casting device as claimed in claim 35,wherein the at least one gas jet nozzle comprises a plurality gas jetnozzles secured to the covering hood and oriented thereby.
 37. Thetwo-roll casting device as claimed in claim 25, wherein the distancebetween the gas jet axis and the first contact line is in a range from10 mm to 50 mm, measured on the bath surface.
 38. The two-roll castingdevice as claimed in claim 26, wherein the angle α is from 25° to 140°.39. The two-roll casting device as claimed in claim 26, wherein theangle α is from 35° to 90°.